Prior art fuel injectors of the type to which the present invention finds application include an injector body having a fuel pressurization chamber that opens to a fuel supply passage and a nozzle supply passage. The injector body also has a nozzle chamber that opens to a nozzle outlet and the nozzle supply passage. A portion of the fuel supply passage is a return spring cavity. A needle check is positioned to reciprocate in the nozzle chamber between a closed position that closes the nozzle outlet and an open position that opens the nozzle outlet. One end of the needle check is exposed within the return spring cavity, and a return spring is compressed in the return spring cavity between the one end of the needle check and an end wall of the cavity. The return spring serves as a means by which the needle check is biased toward its closed position. The needle check includes at least one hydraulic lift surface exposed to the nozzle chamber such that the needle check will move to an open position against the action of the return spring when sufficient hydraulic pressure acts on the hydraulic lift surfaces. A stop pin is positioned within the return spring cavity, and serves as the means by which the movement of the needle check is limited. In other words, the pin acts as a stop and limits the distance that the needle check can move against the action of the return spring. These various components are normally centered about an axis such that the return spring surrounds the pin, which is aligned with the central axis of the injector.
In such prior art fuel injectors, the stop pin is a separate piece and is free to move within the return spring cavity, constrained only by the inner diameter of the return spring and the end walls of the cavity. In other words, the stop pin must necessarily have a smaller diameter than the inner diameter of the return spring in order to minimize interference between the two, and there must be some up and down movement available in order to allow the needle check to open during an injection event. Over time, this freedom of pin movement can cause several problems that often result in a shortened working life for an individual injector. One of these problems occurs when the pin becomes cocked or tilted in its orientation such that the stress is concentrated at the corner edges of the pin when the needle check opens. As the injector goes through numerous injection cycles, this tilted or cocked orientation of the pin stop can result in chipping and/or deformation of the normally plainer stop surfaces that contact one another when the needle check opens. Also loose metal fragments caused by pin chipping can become lodged at locations within the injector that cause a significant disruption or more serious malfunction in the injector. Finally, the deformation and excessive wear caused to the pin and its associated contact surfaces can also undermine and/or disrupt the performance of the injector.
Over time, excessive wear on the return spring can also occur because of one or more reasons, such as spacer tilting. This wear on the return spring can eventually weaken the spring and change the valve opening pressure of the needle check. The valve opening pressure can also be altered by tilting of the return spring itself. Those skilled in the art will appreciate that a change in the valve opening pressure can have a significant affect on injector performance, particularly injection mass flow rate and amount. Eventually, the unnecessary wearing on the return spring can cause spring breakage which results in a catastrophic malfunction in the injector.
Still another problem identified in prior art fuel injectors of this type is cavitation damage to the return spring. This phenomenon is believed to occur when the needle check is moving from its open position to its closed position under the action of the return spring. When this happens, the volume of the return spring cavity necessarily increases with the movement of the needle check. Because the pin stop and the return spring take up a substantial portion of the available volume in the return spring cavity, only a limited volume is available for fluid transfer. This limited fluid volume availability becomes acute when the needle check moves toward its closed position. In other words, negative pressure is created within the return spring cavity during the. abrupt volume increase that occurs when the needle check moves toward its closed position. This abrupt drop in pressure briefly causes cavitation to occur primarily in the neighborhood of the end coils of the return spring. Over time, this cavitation causes excessive wear on the return spring causing the same to be weakened or otherwise damaged. This weakening of the return spring due to cavitation can also undesirably alter the valve opening pressure of the needle check.
The present invention is directed to overcoming these and other problems associated with the nozzle assemblies for fuel injectors.